FIG. 1 is an illustration of several related art nuclear fuel bundles 10 and core components commonly encountered in existing nuclear power technology. As shown in FIG. 1, one or more fuel bundles 10 containing several individual fuel rods may be placed within a reactor core in conventional fuel placement strategies. A channel 20 may surround the fuel rods in each bundle 10, providing directed coolant and/or moderator flow within bundles 10 and/or facilitating manipulation of bundles 10 as a single rigid body. Control rods or cruciform control blades 60 may be extended from set core locations between bundles to absorb neutrons and control reactivity and ultimately control reactivity by a degree of insertion or withdrawal from between the fuel bundles 10. Fuel support 70 may support and align bundles 10 at constant positions within the core.
FIG. 2 is a quadrant map of a related art Boiling Water Reactor (BWR) core, illustrating fuel bundle locations within one quarter of the core. Reactor cores typically are conveniently symmetrical about at least two perpendicular axes, such that a quadrant map of FIG. 2 conveys the makeup of the entire core. As shown in FIG. 2, individual bundle locations are occupied by fresh (shown with diagonal or cross-hatched fill) or burnt (shown with no fill) fuel bundles at the start of a fuel cycle, before commencement of power operations in the core. Fresh bundles are bundles that have not previously been exposed to neutron flux during power operations, i.e., never been burnt, whereas burnt fuel bundles have received such exposure, typically over one or more fuel cycles lasting 1-2 years. As such, burnt fuel bundles typically have exposure, or burnup, of several GWd/ST.
Fresh fuel bundles may have different starting enrichments of fissile material content. For example, in some BWR designs, outer-enrichment bundles (shown in cross-hatched fill) may include approximately 4.3% Uranium-235 fuel, and inner-enrichment bundles (shown in diagonal fill) may include approximately 4.2% Uranium-235 fuel. Varying enrichments, such as the one shown in FIG. 2, may permit a flatter radial power profile in the core and/or achieve other operational effects. Further, in some BWR designs, bundles may also possess varying distributions and concentrations of burnable poisons/neutron absorbers to suppress reactivity and optimize operational characteristics. As shown in FIG. 2, at startup related art nuclear fuel cores include an outer peripheral ring of stale fuel bundles surrounding an inner peripheral ring of fresh, high-enrichment fuel bundles. A central region may include 50% or more fresh bundles in order to maximize fresh fuel content over an even distribution, permitting longer operating cycles with lower downtime.
In related art BWRs, cruciform control blades 60 extend centrally between groupings of four fuel bundles in order to absorb neutrons and control the nuclear chain reaction in the core. As shown in FIG. 2, the groupings of four fuel bundles, between which control blades extend, are identified in bolded outline as controlled bundles, or control cells. Bundles within the controlled bundle groups conventionally have one face closest to a control blade used during the fuel cycle; such bundles are referred to as controlled bundles and their positions as controlled positions in control cells of four bundles. Different control blades in different control cells, usually four or five per quadrant, are conventionally alternately inserted and withdrawn in different and complex control blade sequences in order to manage reactivity and power distribution and spread control blade usage across several different blades and fresh fuel bundles within the core.
As shown in FIG. 2, in order to maximize the number of fresh fuel bundles used in a longer cycle over an even core distribution, several fresh fuel bundles may be placed in controlled positions adjacent to operated control blades within the inner portion of the core. Due to conventional operation of control blades, all fresh fuel bundles in the central core portion may be controlled—having direct exposure to control blades actively moved to finely control reactivity—throughout an entire fuel cycle. Use of fresh fuel bundles in controlled locations causes several problems, including corrosion and channel bowing that worsens in later cycles, and a need to perform complex and/or lower-power control blade sequence exchanges due to this positioning that worsen plant economics. Some related art fuel cores have avoided this problem by using a Control Cell Core loading strategy, where only burnt fuel bundles are placed closest to operated control blades, resulting in fewer fresh bundles used in the central portion of the core and shorter operating cycles.